This type of quadrupole type mass spectrometer is sometimes called by another name, such as a residual gas analyzer, a partial pressure gauge or a mass filter. A conventional quadrupole type mass spectrometer will be described with reference to FIG. 10.
As a method for measuring a gas density (pressure) remaining in a vacuum device 9 in FIG. 10, it uses a total pressure gauge G for measuring the total pressure and a partial pressure gauge Q′ for measuring a density by a type of gas, and the vacuum device 9 is generally provided with both of them.
At present, it is general to use an ionization vacuum gauge (G) as the former total pressure gauge capable of measuring a whole region of high vacuum, ultrahigh vacuum and extra-high vacuum, and a quadrupole type mass spectrometer (Q′), which is provided with an electron impact ion source, as the latter partial pressure gauge. Both of them generally have a hot cathode type filament for an electron emitter.
In the ionization vacuum gauge G (Beyard-Alpert type ionization vacuum gauge, hereinafter referred to as “BA type” in FIG. 10), electrons emitted from a hot cathode filament (hereinafter referred to as “filament”) 3′ which is biased 33′ from ground potential to positive electric potential between 20 to 100 volts are accelerated toward a grid electrode 2′ which is biased 22′ to an electric potential much higher by about 120 volts than the filament's potential, passed through the grid electrode 2′ after having been accelerated, reflected on the other side after having passed through, and oscillate inside and outside of the grid electrode 2′. In the process of oscillation, the electrons partly collide with the grid electrode 2′ and are absorbed by it. At this time, the electrons lost by the grid electrode 2′ are always compensated from the filament 3′, so that the constant electrons are always oscillated within and outside of the grid electrode in the ionization vacuum gauge G.
The oscillating electrons collide with the residual gas molecules in the vacuum device 9 flowed into the grid electrode to generate positive ions within the grid electrode. The positive ions are collected to a needle shape collector electrode 7′ and flowed into an electro-meter 8′ held at the ground potential, and the intensity is measured. This current is proportional to residual gas molecular density (pressure) P, and ion current (signal current) Ii to P is expressed as follows:Ii=SIeP  Equation (1)where, S (Pa−1) is a proportional constant which is called a sensitivity coefficient, and Ie is electron beam current which collides with the grid electrode. In other words, the pressure in the vacuum device can be determined by measuring Ii.
Meanwhile, in a case of the quadrupole mass spectrometer Q′, an ion source 10 is constructed with an ion focusing electrode 4, a grid electrode 2 and the hot cathode filament 3. The ion source 10 has an open-end-cylindrical (BA type) grid and a plate ion focusing electrode 4, which has a hole slightly larger than the diameter of the grid and which is formed at the center, disposed to form a demarcation space A.
Besides, a total pressure measuring electrode 5′ which has a hole r slightly smaller than the hole h of the ion focusing electrode is disposed outside of the ion focusing electrode 4 and connected to an electrometer 50 through a vacuum terminal on the atmosphere side. In other words, in the quadrupole mass spectrometer Q′, the total pressure measuring electrode 5′ provides the same role as the collector electrode 7′ of the ionization vacuum gauge G.
The ions produced in the demarcation space A are attracted to and focused to the ion focusing electrode 4, accelerated toward the total pressure measuring electrode 5′, and partly lost by colliding to the total pressure measuring electrode 5′. The remaining beam current passes through the center hole r formed in the total pressure measuring electrode 5, and flowed as ion beam B to the other side. Therefore, where an electrical lead 51 is connected to the total pressure measuring electrode 5′ and also connected to the electrometer 50 which is held at a ground potential, the pressure in the vacuum device 9 can be determined from the remained (1−k) ion current by subtracting a ratio k (k<1 here) flowed as the ion beam B by the following Equation in the same manner as the ionization vacuum gauge G.Ii′=(1−k)SIeP  Equation (2)
The ions taken out as the ion beam B at the ratio k enters a quadrupole mass analyzing portion 6 (hereinafter referred to as “quadrupole”), separated depending on the mass of the ions, entered into a detector 7, i.e. electron multiplyer, and determined for a strength by mass by an electrometer 8.
But, the ion transmittance through a quadrupole 6 is only about several percents of that of incident ions (ions of the same mass), so that the separated ion current becomes very small. Where a pressure is high and ion current is large enough, it is possible to measure the current as it is by the electrometer 8. But, if the pressure lowers and the ion current intensity becomes 10−10 A or less, amplification of the electrometer becomes difficult. In this case, ion beam B′ is connected to a secondary electron multiplier E which is disposed within the detector 7 which converts the ion beam B′ into an electrical signal, thereby to amplify the ion beam B′ to 100 to 10000 times for once on the vacuum side by using the electron avalanche phenomenon, and after the amplification, it is led to the electrometer 8, and an ion current intensity according to the mass is obtained.
Therefore, both a total pressure and a partial pressure can be measured by the quadrupole mass spectrometer Q′ only, so that it is not necessary to mount both the ionization vacuum gauge G and the quadrupole mass spectrometer Q′ in the vacuum device 9, and the object can be achieved sufficiently by only the quadrupole mass spectrometer Q′. But, it is general to mount both of them on the vacuum device 9. Its reasons will be described separately according to the phenomenon.
The ion beam B from the ion source 10 in FIG. 10 enters the quadrupoles 6, the voltage applied to the quadrupoles 6 varies, only ions corresponding to a detection mass m pass through the quadrupoles 6 and is amplified by the multiplier E, and strength corresponding to the mass m is detected by the electrometer 8. But, the quadrupole mass spectrometer has a drawback that the ion current decreases at a ratio of 1/m to 1/√{square root over ( )}m as the mass number m increases. Besides, an amplification factor of the multiplier E also tends to decrease as the mass number m increases. Because of the two mass differential phenomena, a total of electrometer of the individual spectra obtained from the electrometer 8 and the value of the total pressure electrometer 50 are largely different depending on the gas compositions and are not in a proportional relationship.
In addition, where the multiplier is used in the detector 7, a multiplication factor lowers depending on bakeout times and a repetition, so that it becomes impossible to understand at all to which pressure the peak intensity obtained from the spectrum of the quadrupole mass spectrometer Q′ corresponds in terms of the absolute pressure (an intensity ratio between the individual spectra involved in the pressure change is same). What assists it is an ion current signal which is obtained through the total pressure measuring electrode 5′, an absolute pressure is read by the total pressure measurement, and it is necessary to keep compensating the gas composition ratio of the absolute pressure, and the quadrupole mass spectrometer Q′ is provided with the total pressure measuring electrode 5′.
Because, the object of the quadrupole mass spectrometer Q′ is gas analyzing, and effectively usable ion current is a ratio k (about k<½ here) of the generated ions in the ion source and becomes much smaller by passing through the quadrupoles 6, so that it is necessary to increase the ion transmittance k of the generated ions in the demarcation space A within the ion source 10 as high as possible. Therefore, the ion source 10 which is mounted on the conventional quadrupole mass spectrometer Q′ needs to adjust the potential of the ion focusing electrode 4 to the optimum value. At that time, the ion transmittance k changes, and it becomes impossible to determine the true pressure by the Equation (2).
In addition, an ion distribution density generated in the demarcation space A formed of the grid electrode 2 and the ion focusing electrode 4 changes when the pressure in the vacuum device increases and the ion density increases, and the value (1−k) also changes, and the ion current obtained from the total pressure measuring electrode 5′ deviates from the proportional straight line of the pressure.
Besides, there are the following problems. The ion source 10 mounted on the conventional quadrupole mass spectrometer Q′ is required to have the grid electrode 2, the ion focusing electrode 4, the total pressure measuring electrode 5′ and the quadrupole casing 56 assembled to have a small distance of about 1 mm to 2 mm among them, and individual electric potentials are also different considerably. Therefore, an actual quadrupole mass spectrometer Q′ adopts a structure that ceramic washers 52 and the electrodes 2, 4, 5′ are alternately stacked on a ceramic pipe 53 to satisfy both a distance and insulation as shown in FIG. 11, and a different bias is applied to the individual electrodes. Generally, the grid electrode 2 is biased to 220V, the ion focusing electrode is biased to 200V, and the total pressure measuring electrode 5′ is biased to the same ground potential (0V) as the quadrupole casing 56.
But, alumina ceramic has an insulation resistivity of about σ=1014 Ω·cm at 20° C., but the temperatures of the electrodes and ceramic parts around the ion source 10 are increased to about 100° C. by heat from the hot cathode filament 3. Therefore, the resistivity of the ceramic lowers to σ=1013 Ω·cm or less. For example, when it is assumed that the ceramic between the ion focusing electrode 4 and the total pressure measuring electrode 5′ has a thickness of 1 mm and supported at three portions, a total area of the washer type ceramic insulator 52 becomes about 1 cm2, and the total resistance becomes R=1×1012 Ω. And, leak current L of about I=V/R=200/1×1012=2×10−10 A is generated between the ion focusing electrode 4 and the grid electrode 2. Spurious pressure P generated by the leak current L can be calculated by using the Equation (2), and the pressure is expressed as follows when it is assumed that the sensitivity coefficient is S=1×10−2 Pa, electron current is Ie=2×10−3 A, and a ratio of the ion beam B is k=0.7:P=L÷[(1−k)SIe]=2×10−10÷[0.3×10−2×2×10−3]3.3×10−5 Pa.It is when the insulating ceramics 52, 53 are completely free from contamination and in an ideally insulated state. In practice, the total pressure which can be measured by using the ion focusing electrode 5′ is limited to a high pressure of 10−5 Pa or more by an influence of the leak current.
Meanwhile, in a case where the total pressure is measured by means of the conventional quadrupole mass spectrometer Q′ shown in FIG. 10, there is a problem of a quadrupole fringe field. Among the quadrupoles 6, mutually crossing two are short-circuited to provide two electrodes, these two electrodes have AC voltage of Vcosωt overlapped with ±U DC voltage, scanning is performed depending on the mass m such that U/V becomes always constant (when m is small, U is also small, and when m is large, U also becomes large), and an electric field is accordingly applied to the four quadrupoles 6. Generally, kinetic energies of ions to be entered into the quadrupoles 6 must be decelerated to 10 electron volts or less, so that the center electric potential of the quadrupoles 6 is close to the electric potential of the grid electrode 2, and it is held at electric potential higher by 200V or more than the total pressure measuring electrode 5′ of the ground potential. If the analysis mass m is large, a high voltage of about 300 to 400V is present on the back side of the total pressure measuring electrode 5′. Therefore, the ion beam B which has left the ion focusing electrode 4 is once accelerated to the maximum by the total pressure measuring electrode 5′, and immediately after the ion beam B passes through the hole r of the total pressure measuring electrode 5′, the electric field works so that the ion beam B is reflected at the inlet of the quadrupoles 6.
Therefore, the same ions are partly reflected (hereinafter referred to as “quadrupole fringe field problem”) at the inlet of the quadrupoles 6, and the reflected ions from the opposite side of the quadrupoles 6 flow into the total pressure measuring electrode 5′. The reflected amount is variable depending on the mass m, so that there is a large difference in the total pressure measurement depending on the ion compositions.
To solve the above-described quadrupole fringe field problem, a quadrupole mass spectrometer Q″ is proposed to disuse the total pressure measuring electrode 5′ of FIG. 10 by using an electronic repeller electrode 57 shown in FIG. 12, and it has become known (Japanese Patent Laid-Open Publication No. Hei 7-037547).
But, this known method has more defects than the method using the above-described total pressure measuring electrode 5′. The reasons will be described with reference to FIG. 13 which shows a sectional view of a part of the ion source 10 when the electronic repeller electrode 57 of FIG. 12 is used as a total pressure measurement electrode.
In FIG. 13, the electronic repeller electrode 57 is disposed to surround the cylindrical grid electrode 2 and the circular filament 3. Electrons having come out of the filament 3 are accelerated by the grid electrode 2 to burst out to the opposite side, reflected by the electronic repeller electrode 57 and repeat oscillating in and out of the grid electrode to collide with the gas molecules to produce ions. The ions are generated not only in the grid electrode 2 but also in a portion c between the grid electrode 2 and the electronic repeller electrode 57. The electronic repeller electrode 57 is positioned on a ground level and connected to the electrometer 50. In other words, the ions generated between the grid electrode 2 and the electronic repeller electrode 57 can be pulled toward the electronic repeller electrode 57 to be measured, and this current is proportional to the pressure, so that the same Equation (1) can be used to determine the pressure (the value of sensitivity S is different). It is described in the Japanese Patent Laid-Open Publication No. Hei 7-037547 that because the total pressure can be measured by the electronic repeller electrode 57, an influence of reflecting of the ions by the quadrupole fringe field is not caused, and accurate pressure measurement can be made.
But, the above method also has two great problems. One is that electrons repeat oscillating in and out of the grid electrode 2 but finally collide with the grid electrode 2 as described above. The electrons have an energy of about 120V when they collide with the grid electrode 2, so that a soft X-ray corresponding to about 1/105 of the colliding electrons is generated as x from the surface of the grid electrode 2. This soft X-ray's x is absorbed by the electronic repeller electrode 57 which surrounds it.
But, about 1/100 of the absorbed soft X-ray's x is emitted as photoelectrons e from the electronic repeller electrode 57 by a photoelectric effect. In other words, with respect to the electrons which collide with the grid electrode 2, the electrons corresponding to 1/107 of the current are generated from the electronic repeller electrode 57. Flowing of the ions into the electronic repeller electrode 57 and the generation of the electrons from the electronic repeller electrode 57 are in the same direction as the direction of the electrometer 50, so that a value corresponding to the current according to the X-ray photoelectric effect, namely a spurious pressure is shown. This is a phenomenon which occurs even if the ions do not flow (gas molecules are eliminated) into the electronic repeller electrode 57.
It was first found in the U.S. in the 1940s that this phenomenon results from the fact that the pressure indicated by a triode type (a hairpin filament, a cylindrical spiral grid electrode, and a cylindrical collector surrounding it) ionization vacuum gauge does not decrease to 10−6 Pa or less. To improve it, the conventional BA type ionization vacuum gauges G shown in FIG. 10 and FIG. 12 were provided. This phenomenon is called an X-ray limit of the ionization vacuum gauge. An idea of using the electronic repeller electrode as an ion collector means a return to the same structure as the triode type ionization vacuum gauge. When it is assumed that electron current is Ie=2 mA, the sensitivity of the electronic repeller electrode 57 can be estimated as about S=0.05/Pa. And, when it is assigned to the Equation (1), spurious pressure Px according to the soft X-ray can be estimated as follows:Px=Ii/SIe=(Ie×10−7)÷SIe=10−7÷S=2×10−6 (Pa),and a pressure lower than it cannot be measured.
Besides, a second problem involved when a total pressure is measured by the electronic repeller electrode 57 is that positive ions (such as alkali metal ions) j generated from the hot cathode filament 3 cannot be prevented from entering the electronic repeller electrode 57 because the hot cathode filament 3 is present in an ion generation space c. The positive ions j generated from the hot cathode filament 3 are also ions not related to the pressure, and even if their generation eliminates the gas molecules, the value indicated by the electrometer 50 does not decrease because of the entry of the ions. Meanwhile, the positive ions j generated from the filament cannot enter the grid inside in the grid electrode 2 in view of the electric potential, so that the total pressure measuring method using the conventional total pressure measuring electrode 5′ of FIG. 10 does not have a problem of the positive ions j generated from the filament.
As apparent from the description about the problem of measuring the pressure in the vacuum device 9, the measurement by the conventional quadrupole mass spectrometers Q′, Q″ is limited to a relative proportion among the gas components of the residual gas, namely the partial pressure only, and it is quite difficult to determine its absolute value. To assist it, another ionization vacuum gauge G for accurately determining the absolute pressure of the whole is required. Especially, the existing vacuum device 9 used at a pressure of ultrahigh vacuum of 10−5 Pa or less required the measuring devices such as both the quadrupole mass spectrometer Q′ or Q″ and the ionization vacuum gauge G. For the quantitative analysis of the partial pressure, it was necessary to perform the qualitative gas analysis by the quadrupole mass spectrometer and disperse the value obtained from the ionization vacuum gauge to the ratio obtained by the quadrupole mass spectrometer.
But, even if both the measuring devices are mounted on the same vacuum system 9, outgassing speeds from the two devices difference largely between the quadrupole mass spectrometer Q′ or Q″ and the simple-structured ionization vacuum gauge G in an ultrahigh vacuum region of 10−7 Pa or less, so that the obtained partial pressure and total pressure often indicate largely different values. Therefore, even if two measuring devices were prepared, there were problems that their functions could not be exerted sufficiently, and mounting two of them was uneconomical.
[Patent Document 1]
Japanese Patent Laid-Open Publication No. Hei 7-037547
The problems to be solved by the present invention are as follows:    (1) In a case where a pressure is measured with the total pressure measuring electrode, sensitivity is variable in a pressure region or by fine adjustment of the electric potential between the electrodes within the ion source.    (2) In a case where a pressure is measured by means of the total pressure measuring electrode, a measuring limit remains at 10−5 Pa due to the leak current between the electrodes.    (3) In a case where a pressure is measured by means of the total pressure measuring electrode, a problem of quadrupole fringe field occurs.    (4) In a case where a total pressure is measured by means of the electronic repeller electrode, an X-ray limit is high.    (5) In a case where a total pressure is measured by means of the electronic repeller electrode, a disturbance is caused by positive ions from the filament.    (6) In a case where a total pressure is measured by means of conventional quadrupole mass spectrometers, all the measuring limits are about 10−6 Pa.    (7) In a case where a pressure of the vacuum device is measured, both the quadrupole mass spectrometer and the ionization vacuum gauge are required.    (8) There is a difference between a value obtained by measuring the total pressure with the ionization vacuum gauge and a total value obtained by measuring the partial pressure with the quadrupole mass spectrometer.
The present invention has been made to solve the above-described problems (1) to (8).